A giant in the field takes stock of his science.
Few people would have the credentials, history, background, or brazen confidence to write a book with so definitive a title as What Evolution Is. But few would deny the right of Ernst Mayr, S.D. '80, Agassiz professor of zoology emeritus, to produce such a volume. Former director of Harvard's Museum of Comparative Zoology (MCZ), winner of a galaxy of international prizes, Mayr's major contributions to the field of evolution stretch back well over 60 years.
|Ernst Mayr (right) and his Malay mantri (field assistant), Sario, emerging from two months of surveying birds in the interior mountains of New Guinea in 1928--a pioneering study of species formation.|
|Photograph courtesy Ernst Mayr and the Ernst Mayr Library, Harvard Museum of Comparative Zoology|
Born in 1904, he is the last surviving member of a cadre of biologists who shook the world with their stunning blend of genetics, mathematics, natural history, and paleontology in the 1930s, '40s, and '50s. This group produced what is now called the "evolutionary synthesis," a brilliant and far-reaching set of papers, books, and discoveries that has charted the path of modern evolution ever since.
Now, more than half a century after Mayr's initial contributions to that advance, we have a chance to see what this icon of biological innovation thinks about the use we've made of the synthesis, what it has accomplished, and where it is going. What Evolution Is does not dwell on historical developments or "who said what." Instead it takes all that for granted and tries to lay out for the nonscientific reader the power, accomplishments, and thrill of modern evolutionary science--a science, he notes, that we apply "to cope with antibiotic resistance by pathogens, pesticide resistance by crop pests, the control of disease vectors (e.g., malaria mosquitoes), human epidemics, the production of new crop plants by evolutionary genetics, and many more challenges."
Mayr has a lot of perspective to draw from, having published major evolutionary works--works that everyone in evolutionary science must read--every 20 years, and scores of other books, papers, monographs, and commentaries in between. He has contributed a great deal of hard data about evolution, but he has also arrived at critical insights and historical discoveries about how people over the past century have learned to think about evolutionary change. Mayr describes one of these key tools as "population thinking": the ability to think about populations of organisms and how they thrive, rather than thinking merely about individuals, the way a zookeeper might, or species, as a museum curator might. Mayr himself exemplified the value of a curator: understanding the natural setting and intrinsic biological variability of species. Through his research on bird species and subspecies in the Asian tropics, Mayr realized the critical need to integrate nonmorphological information about species into the growing understanding we have of their evolution. Where do new bird species come from? Mayr would answer by inquiring about mating patterns and ecological differences and the geography of color-plumage variants.
In 1941, he received a prestigious invitation to present two Jesup Lectures at Columbia University--then a world powerhouse of evolutionary biology. The success of those lectures led naturally to Systematics and the Origin of Species, published in 1942--a book that would become one of the pillars of the evolutionary synthesis. Already one of the world's authorities on the taxonomy and classification of birds, Mayr also represented the vibrant heart of the rebirth of museum-based scientific research. He had realized that cradled in the careful collections of the American Museum of Natural History in New York City, one of the world's most venerated zoological collections, where he was curator of birds from 1931 to 1953, lay a store of knowledge about natural history that was pivotal in the emerging understanding of the evolutionary process.
Natural history celebrates the geography, morphology, behavior, life styles, and ecology of the world's species by unraveling their life stories and seeking to make sense of their interactions. Previously the pursuit primarily of wealthy gentry with their shell collections and plant presses, natural history in the twentieth century became one of the missions of major museums around the world. Stored in vaults, pinned in cabinets, displayed in drawers or rows of glass bottles, species from around the planet were at hand to reveal their stories.
Mayr himself had uncovered many new species, particularly of the birds of New Guinea and the Pacific. But he knew there was more information in a museum than just its species lists. By adding geographic information to those lists, Mayr realized that he could chart each species' position relative to the positions of similar species. His maps told him a simple but powerful fact: the most closely related species--for example, those that differ by just a few colored feathers--usually had close but non-overlapping distributions. (A map from Systematics graphically illustrates the point, showing the separate ranges of upland species of New Guinea birds of paradise, genus Astrapia, whose brightly colored males differ in plumage--evidence of rapid evolution of mating displays among diverging species.) Species that were more different, those that had diverged from one another long ago, often had such overlapping distributions that individuals of both species regularly inhabited the same places.
From these simple maps, and from other data drawn from the natural history of species, Mayr proposed that most species arose from geographically separated populations of one species. The new, young species start out with non-overlapping ranges because the original populations that formed them were separated, but over time the diverging species spread out and come back into contact. By then, these species are so different that they can't or won't interbreed, and the multiple species persist. Forcefully promoted and extensively documented by Mayr throughout the last half of the twentieth century, geographic speciation--now called allopatric speciation--has reigned for 60 years in evolutionary theory as the pre-eminent mode of species formation.
Mayr's views grew to dominate our understanding of species formation, but What Evolution Is generously offers perspective on new ideas since his breakthrough documentation of the geographic basis for species formation in Systematics. In this new book, Mayr discusses the success of competing theories that de-emphasize geography and allow for the formation of species without any prior geographic separation. The key observations of Guy Bush, Ph.D. '64--made while Bush was a graduate student--as well as new information about species flocks in African lakes, provide some of the best evidence for so-called sympatric (e.g. "living together") speciation.
Besides discussing species formation, Mayr provides vignettes and brief commentary on many other issues bubbling up in current evolutionary science. Development of animal body plans and how this delicate process evolves occupies the imagination of many of evolution's key thinkers. Mayr offers a summary of this research, including a few pivotal observations--like the fact that similar genes are involved in eye formation in animals with extraordinarily different eyes, such as insects and mammals. We also see Mayr engaging other issues outside his normal ambit, like the evolution of genomes and the patterns of molecular evolution that shape them. Here, as elsewhere, Mayr can be refreshingly clear and direct: "Perhaps the most unexpected result of modern molecular studies of the genome was the discovery of the great age of many genes. The sequence of base pairs is often so conservative that one can determine that a certain mammalian gene is also part of the genome of the fruit fly Drosophila melanogaster or the nematode Caenorhabditis. Indeed, it seems possible to trace some genes all the way from animals or plants to bacteria."
Human evolution does not escape scrutiny, and Mayr offers his own interpretations in this fast-moving and perennially controversial field: "What is perhaps most astonishing is the fact that the human brain seems not to have changed one single bit since the first appearance of Homo sapiens, some 150,000 years ago."
Mayr's new book is not a textbook, and so he returns often to a few keystone controversies--such as which line of dinosaurs gave rise to the modern birds--in order to keep from confusing the general reader with too many disparate examples. Nor is this book a comprehensive exposition of current evolutionary issues. Its conversational tone may seem unstructured sometimes, but it is hauntingly similar to actually sitting in Mayr's MCZ office--still stacked with his many books and specimens--and being regaled by Mayr himself with opinions, ideas, and historical fact.
In that sense, What Evolution Is lays out the perspectives and views of one of evolutionary science's most recognized figures, informing us all how one of the original architects of the evolutionary synthesis thinks about modern biology, as a science and in its wider reaches. What would Mayr say about human races or the Cambrian explosion? What does he think about the persistent creationists who are trying to confuse the teaching of science to America's school children? What is the next step that Mayr expects evolutionary scientists to take? What Evolution Is serves as readers' best chance to catch up on what this academic giant has to say.
Stephen Palumbi, professor of biology and curator of invertebrates in the Museum of Comparative Zoology, is the author of The Evolution Explosion.